<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>

        .demo {
            background-color: orange;
        }
    </style>
</head>
<body>

<button id="btn">点我创建一个输入框</button>


<script>
    const btn = document.getElementById('btn')

    btn.onclick = () => {
        const input = document.createElement('input')
        // input.focus() //不会生效
        input.className = 'demo'
        document.body.appendChild(input)

        input.focus()

        // input.parentElement.style.backgroundColor = 'red'
    }
</script>
</body>
</html>